This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In cellular base stations of 4G and beyond, advanced digital modulation scheme is used for high spectrum efficiency. The radio frequency (RF) signal for 4G and beyond exhibits a large peak to average power ratio (PAPR), which is amplified simultaneously in a power amplifier (PA). Therefore, the instantaneous transmit power will vary very extensively and fast.
Conventionally, a Doherty PA is used to enhance amplification efficiency for high PAPR signal. However, with ever higher PAPR, the main difficulties in the Doherty PA are that Doherty region should be limited to maintain high efficiency with PAPR greater than a certain range, e.g. Doherty region should be limited so that the Doherty PA has output power back off with 6 dB.
A conventional Doherty PA comprises traditional amplifier units or cells that exhibit limited efficiency characteristics which defines the upper limit of the achievable efficiency of the Doherty PA. Various theories of operation of high efficiency amplifiers have been developed over the years to address the need for achieving high efficiency in amplifiers of Doherty PAs for high PAPR signal.
The conventional solution had maximum drain efficiencies exceeding 70%. However, low losses are particularly critical in the high power circuits where the dissipated power not only drains the power source but often substantially increases the junction temperature of the device. High operational temperatures thereby lead to lower performance and lower reliability of the Doherty PAs.
Harmonic injection method is a valid approach to boost amplifier cell peak power efficiency and efficiency around the peak power. In harmonic injection power amplifier, the harmonic is injected into the input terminal of the power amplifier, and efficiency of the power amplifier will increase to a certain extent. In prior art, the harmonic is usually generated and injected by active harmonic injection apparatus.